This invention relates to a method of manufacturing fluorescent lamps.
Lately, energy costs have steeply risen due to a great drain upon resources, environmental problems etc. so that it is required in industrial circles to reconsider the conventional productional structure aimed principally at mass production and to improve the overall production efficiency by taking account of the saving of resources and energy. In order to meet this requirement of the times, various countermeasures have also been taken in the production of fluorescent lamps.
For example, as solvents for the phosphor suspension, expensive organic solvents have been previously employed, but as an example of the countermeasures, there can be mentioned the developement of a method using water which is abundant in the sense of resources and also inexpensive.
However, this method, that is, the method of forming phosphor coatings by using an aqueous phosphor suspension has the disadvantage that, when used with high speed mass production apparatus, a great obstruction is caused by bubbling which has not been a problem with organic solvents so that an increase in defective coatings and the deterioration of the exterior quality due to the generation of bubbles cannot be avoided completely.
On the other hand, what consumes the most energy among the steps of manufacturing fluorescent lamps is the step of baking the phosphor layers. This step aims at the decomposition and removal of a binder which is a high molecular weight organic material required for the formation of phosphor films, by heating and baking it after the particular phosphor layer has been formed and the heating temperature is determined by the decomposition characteristics of the binder used. Where the phosphor layer is formed by using an organic solvent, either nitrocellulose or ethylcellulose is used as the binder. In order to decompose it completely, heating at 600.degree. C. or more is required, such a temperature approximating the softening temperature of the glass bulbs used. As a result, in the step of baking the phosphor layer, there is adopted a system by which a glass bulb rides on a pair of juxtaposed metallic rollers having surfaces coated with a refractory material and passes through a heating furnace while the rollers are rotating. Therefore, a heat loss due to the rollers is extremely large, thus greatly decreasing the thermal efficiency of the heating furnace. This causes a large amount of thermal energy to be consumed.
In order to reduce a quantity of energy consumed in this baking step, it is the indispensable requirement to improve the thermal efficiency of a baking device. The most effective method is to use a binder which can be completely decomposed and removed at a low heating temperature so as not to deform the glass bulb and to make a system not using rollers which are high in heat capacity as the means for transferring glass bulbs through the baking device. However, with phosphor layers formed of the phosphor suspension by using an organic solvent, there are not suitable binders which meet the above-mentioned requirement, and accordingly there has not been found effective means for reducing energy in the baking step.
However, as the use of water soluble phosphor suspensions has recently been generalized as described above, a range within which binders can be selected becomes very wide and various binders or water-soluble high molecular weight organic materials have been discussed. Among them it has been possible to use those binders which are completely decomposed and removed by heating to a temperature on the order of 500.degree. C., for example, using polyethylene oxides or fatty acid esters thereof as binders. When phosphor layers are formed with aqueous lacquers including such binders, the baking temperature can be selected to be somewhat lower than a temperature at which the associated glass bulbs are deformed (i.e. below the softening temperature of the glass). This eliminates a fear that the glass bulbs will be deformed during the baking step. Accordingly, it is possible to use a baking furnace which is quite different from those of the conventional type. In the conventional prior art baking step, the decomposition and removal of the binders is completely accomplished but when baked at a temperature on the order of 500.degree. C., the baked phosphor layers have extremely weak adhesion to the glass surface and the percentage of defective lamps increases due to the exfoliation of the phosphor layers. By considering the merits and demerits as a whole, there have arisen new problems in that a loss due to a decrease in yield is rather in excess of the advantage that the amount of energy consumed in the baking step is decreased.
Accordingly, it is an object of the present invention to provide a new and improved method of manufacturing fluorescent lamps which makes it possible to sharply decrease the amount of energy consumed in the step of baking phosphor layers without adversely affecting the quality of lamps and yields in the manufacturing steps during the manufacture of fluorescent lamps.